Decide what you want:
What do you hope to accomplish?
Who will execute on your social media strategy?
How will they do that? What do they need?
Get started with free tools: Define a blog policy, encourage employees to blog,
Start a company blog, link to employee blogs
Start a company twitter account, follow your customers
Start a company facebook group, befriend your customers
If you want to customize your existing website, consider buying a vendor solution: Forrester analysis of social networking platforms, with process explained